This invention generally relates to prepreg and laminate compositions for use in forming printed circuit boards having a reduced and uniform dielectric constant, improved thermal stability and thermal expansion characteristics, uniform appearance, low density and improved drillability.
The printed circuit board (PCB) is central to the electronic systems of modern devices, which typically have high signal speeds and operating frequencies. The circuit boards made of materials with low dielectric constants permit the speed of electronic signal transmission in the laminates to be increased and data to be processed at greater speeds. Thus, by using a printed circuit board with a lower dielectric constant, the system may be designed with a higher speed of processing electric signals. Since the dielectric constant of the material from which the PCB is formed has a direct effect on the performance and speed of circuits built on the board, the increased electrical performance and the demand for increased speed required of PCBs in modern devices has prompted a search for ways to reduce the dielectric constant of the materials from which they are made.
Printed circuit boards are typically produced by impregnating a fabric, such as an electronics grade fiber glass (E-glass) with a liquid thermosetting epoxy resin. The impregnated fabric is heated to partially cure the resin and to form a dry, flexible sheet in which the resin is in an intermediate cure state, sometimes referred to as the xe2x80x9cBxe2x80x9d stage or a xe2x80x9cpre-preg.xe2x80x9d Pre-preg sheets are then stacked together to a desired thickness and subjected to heat and pressure that fully cures the resin. This forms a laminated composite in which the resin is sometimes said to be in the xe2x80x9cCxe2x80x9d-stage.
Typically, printed circuit boards consist of about 50% by weight epoxy and 50% by weight electronics grade fiber glass (E-glass). The typical dielectric constant of the laminate at 50% resin content is about 4.6, a value which is insufficiently low to satisfy the demands of the most high speed computers currently being developed.
To reduce the dielectric constant of printed circuit board, polymeric resins such cyanate esters, polyimides, BT/epoxy, polyphenylene ether, and PTFE which have a dielectric constant which is less than the dielectric constant of epoxy have been considered, but they tend to be relatively expensive or suffer from other disadvantages. For example, PCBs made from PTFE may have a dielectric constant of approximately 2.5 at 1 MHZ, but they are relatively expensive and difficult to manufacture, composites impregnated with PTFE are difficult to fabricate into multilayer printed circuit boards, and pre-preg sheets prepared with PTFE can be bonded only at temperatures at which innerlayers melt and lose their dimensional stability, and they have relatively poor mechanical properties because they are not thermosetting. Laminates prepared using polyphenylene oxide/brominated epoxy resins (PPO/Epoxy) are relatively less expensive than PTFE, but the dielectric constant, Dk, of the laminate, however, is typically about 4.0 and the product is relatively to difficult to consistently manufacture, as the composition of the resin tends to fluctuate from run to run.
Fibers formed from materials other than fiberglass have also been considered as a means to reduce the dielectric constant of the laminate. Aramid fibers together with epoxy resins typically provide laminates with a dielectric constant in the range of about 3 to 4. Although aramid fibers have a low dissipation factor, low mechanical strength, poor adhesive properties and relatively high expense are also associated with these systems. Quartz fibers have also been used, but like aramid fibers they are relatively expensive compared to conventional E-glass. Polyester fibers have a dielectric constant of less than 3, but they tend to suffer from low resistance to heat and a tendency to melt and loose their desirable properties at higher temperatures; polyester fibers has a lower strength relative to glass fiber, which yields less mechanical properties such as dimensional stability.
Okada et al. (U.S. Pat. No. 4,798,762) disclose adding a filler material to resin to reduce the dielectric constant of a laminate in a process in which a hardened plate is extruded and then laminated on opposite sides to preimpregnated reinforcing fibers. According to Okada et al., it is advantageous to use hollow microspheres of alumina, silica, zirconia, glass, carbon and phenol resin. Okada et al.""s preferred filler consists of hollow glass microspheres of 20-150 micrometers in diameter having a glass thickness 0.5-2 micrometers with the volume fraction of filler based on the total volume of the center plate being 0.3 to 0.8, more preferably 0.5 to 0.7. See, U.S. Pat. No. 4,798,762 at col. 3, lines 23-31.
The use of glass microspheres as a filler material to reduce the dielectric constant of the laminate, however, is not without disadvantages. Because the glass shell has a relatively high dielectric constant and somewhat offsets the very low dielectric constant gas which is incorporated within the hollow shell, a relatively high loading of the glass microspheres is required to provide a relatively low dielectric constant laminate. Also, the interface adhesion between the glass microspheres and the resin matrix is often unsatisfactory, leading to thermal, mechanical and distribution problems. Because hollow microspheres are buoyant and relatively hard to disperse, additional equipment for continuous agitation is required to keep them suspended
Chellis et al. (U.S. Pat. No. 5,126,192) disclose that smaller glass microspheres are less buoyant than are larger microspheres. For this reason, Chellis et al. prefer using microspheres having a maximum diameter of about 25 micrometers with a mean diameter of about 5 micrometers. Nevertheless, Chellis et al. require continuous agitation to keep them suspended and suggest using low-shear mixing techniques to minimize damage to the microspheres. See U.S. Pat. No. 5,126,192 at column 4, line 65 to column 5, line 3 and column 6, lines 20-30.
Among the objects of the invention, therefore, is the provision of a prepreg and a laminate which comprises an alternative filler to the traditional glass microspheres, which has a relatively low and uniform dielectric constant, improved thermal expansion characteristics, which minimizes through hole failure, and is flame retardant and easily processable.
Briefly, therefore, the present invention is directed to a filled prepreg composition for use in forming printed circuit boards. The prepreg comprises a reinforcing material impregnated with a cured polymeric resin, the cured polymeric resin comprising multicellular polymeric microspheres as a filler. Incorporation of the multicellular polymeric microspheres advantageously enables the preparation of laminates and printed circuit boards having a relatively low and uniform dielectric constant, improved thermal stability and thermal expansion characteristics, uniform appearance, low density and improved drillability.
Other objects of the invention will be in part apparent and in part pointed out hereinafter.